Work machines such as hydraulic excavators generally control operation of actuators by driving a pump using an engine and by controlling flow rates and directions of a hydraulic fluid supplied from the pump to the actuators. One of devices that control the flow of a hydraulic fluid into an actuator is a flow control valve of an open-center type. When this type of flow control valve is used, under a state where the hydraulic fluid is not supplied to the actuator, the fluid flows into a tank via a bleed-off opening in the valve. When the hydraulic fluid is supplied to the actuator, on the other hand, a meter-in opening in the flow control valve, connecting to the actuator, increases in area and the bleed-off opening correspondingly decreases in area (refer to Patent Document 1).